Active matrix organic electroluminescent display device simplifying fabricating process

ABSTRACT

An active matrix organic electroluminescent display device of the present invention is fabricated through a six-mask process unlike the related art that uses eight masks. In the present invention, since the ground line and the power line are entirely disposed above the substrate, the resistance of the power line is reduced and the thermal damage that may occur in the power line during driving the device is prevented. Therefore, the image quality increases and the uniformity in the display can be obtained. Furthermore, due to the reduction of the mask process, the occurrence of defects is reduced and the production yield can be raised. Additionally, the principles of the present invention can be applied to either the top emission type organic electroluminescent display device or the bottom emission type organic electroluminescent display device. When it is utilized for the top emission type, the active matrix organic electroluminescent display device can have a high aperture ratio.

This application claims the benefit of Korean Patent Application No.2001-0088538 filed on Dec. 29, 2001, which is hereby incorporated byreference in its entirety for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to organic electroluminescent displaydevices, and more particularly, to an active matrix electroluminescentdisplay devices having thin film transistors.

2. Discussion of the Related Art

As an information age has been evolved rapidly, a necessity for flatpanel displays, which have advantages such as thinness, lightweight andlower power consumption, has been increased. Accordingly, various flatpanel display (FPD) devices such as liquid crystal display (LCD)devices, plasma display panels (PDPs), field emission display devicesand electro luminescence display (ELD) devices have been researched andhave been developed.

Among many kinds of FPD devices, the electroluminescence display (ELD)device makes use of electro luminescence phenomenon in which light isgenerated when an electric field of certain intensity is applied to afluorescent substance. The electroluminescence display (ELD) devices canbe classified into inorganic electroluminescence display (ELD) deviceand organic electroluminescent display (ELD) device depending on asource that excites careers. The organic electroluminescent display(ELD) device has drawn attention as a displaying device for naturalcolors because it can display every color in a range of a visible lightand has a high brightness and a low voltage.

In addition, because the organic electroluminescence display (ELD)device is a self-luminescent, it has a high contrast ratio and issuitable for an ultra-thin type display device. Moreover, because it hasa simple manufacturing process, the degree of environmentalcontamination is relatively low. Besides, the organicelectroluminescence display (ELD) device has a few microseconds (μs)response time, so that it is suitable for displaying moving images. Theorganic electroluminescence display (ELD) device has no limit in aviewing angle and is stable in low temperature condition. Because it isdriven with a relatively low voltage between 5V and 15V, a manufacturingand design of a driving circuit is easy.

A structure of the organic electroluminescent display (ELD) device issimilar to that of the inorganic electroluminescence display (ELD)device, but the light-emitting theory of the organic ELD device isdifferent from that of the inorganic ELD device. That is, the organicelectroluminescent display (ELD) device emits light by a recombinationof an electron and a hole, and thus it is often referred to as anorganic light emitting diode (OLED).

Recently, active matrix type of ELD, in which a plurality of pixels isarranged in a matrix form and a thin film transistor is connectedthereto, has been widely applied to the flat panel display devices. Theactive matrix type is also applied to the organic electroluminescentdisplay (ELD) device and this is referred to as an active matrix organicelectroluminescent display (ELD) device.

FIG. 1 is an equivalent circuit diagram showing a basic pixel structureof a conventional active matrix organic electro luminescent display(ELD) device.

In FIG. 1, a pixel of the active matrix organic electroluminescentdisplay device has a switching thin film transistor 4, a driving thinfilm transistor 5, a storage capacitor 6 and a light emitting diode(LED) 7. The switching thin film transistor 4 and the driving thin filmtransistor 5 are comprised of p-type polycrystalline silicon thin filmtransistor. A gate electrode of the switching thin film transistor 4 isconnected to the gate line 1 and a source electrode of the switchingthin film transistor 4 is connected to the data line 2. A drainelectrode of the switching thin film transistor 4 is connected to a gateelectrode of the driving thin film transistor 5, and a drain electrodeof the driving thin film transistor 5 is connected to an anode electrodeof the light emitting diode (LED) 7. A cathode electrode of the lightemitting diode (LED) 7 is grounded. A source electrode of the drivingthin film transistor 5 is connected to a power line 3, and a storagecapacitor 6 is connected to the gate electrode and the source electrodeof the driving thin film transistor 5.

In the pixel structure shown in FIG. 1, if a scanning signal is appliedto the gate line 1, the switching thin film transistor 4 is turned onand an image signal from the data line 2 is stored into the storagecapacitor 6 through the switching thin film transistor 4. If the imagesignal is applied to the gate electrode of the driving thin filmtransistor 5, the driving thin film transistor 5 is turned on and thusthe light emitting diode (LED) 7 emits light. Luminance of the lightemitting diode (LED) 7 is controlled by varying an electric current ofthe light emitting diode (LED) 7. The storage capacitor 6 serves to keepa gate voltage of the driving thin film transistor 5 constant while theswitching thin film transistor 4 is turned off. That is, because thedriving thin film transistor 5 can be driven by a stored voltage in thestorage capacitor 6 even when the switch thin film transistor 4 isturned off, the electric current can keep flowing into the lightemitting diode (LED) 7, and thus the light emitting diode (LED) emitslight until a next image signal comes in.

FIG. 2 is a schematic cross-sectional view of a related art activematrix organic electroluminescent display device. FIG. 2 shows anorganic light emitting diode, a storage capacitor and a driving thinfilm transistor. Moreover, a bottom emission type, in which light isemitted through an anode of a lower electrode, is adopted.

In FIG. 2, a buffer layer 11 is formed on a substrate, and then a firstpolycrystalline silicon layer having first to third portions 12 a, 12 band 12 c and a second polycrystalline silicon layer 13 a are formed onthe buffer layer 11. The first polycrystalline silicon layer is dividedinto the first portion 12 a (i.e., an active region) where impuritiesare not doped and the second and third portions 12 b and 12 c (i.e.,respectively, a drain region and a source region) where the impuritiesare doped. The second polycrystalline silicon layer 13 a becomes one ofthe capacitor electrodes. A gate insulation layer 14 is disposed on theactive region 12 a, and a gate electrode 15 is disposed on the gateinsulation layer 14. A first interlayer insulator 16 is formed on thegate electrode 15 and on the gate insulation layer 14 while covering thedrain and source regions 12 b and 12 c and the second polycrystallinesilicon layer 13 a. A power line 17 is disposed on the first interlayerinsulator 16 particularly above the second polycrystalline silicon layer13 a (i.e., the capacitor electrode). Although not shown in FIG. 2, thepower line 17 extends as a line in one direction. The power line 17 andthe second polycrystalline silicon layer 13 a with the first interlayerinsulator 16 therebetween form a storage capacitor. A second interlayerinsulator 18 is formed on the first interlayer insulator 16, coveringthe power line 17.

Meanwhile, first and second contact holes 18 a and 18 b, which penetrateboth the first and second interlayer insulators 16 and 18, expose thedrain region 12 b and source region 12 c, respectively. Additionally, athird contact hole 18 c, which penetrates the second interlayerinsulator 18, is formed and exposes a portion of the power line 17. Adrain electrode 19 a and a source electrode 19 b are formed on thesecond interlayer insulator 18. The drain electrode 19 a contacts thedrain region 12 b through the first contact hole 18 a. The sourceelectrode 19 b contacts both the source region 12 c and the power line17, respectively, through the second contact hole 18 b and through thethird contact hole 18 c, respectively. A first passivation layer 20 isformed on the drain and source electrodes 19 a and 19 b and on theexposed portions of the second interlayer insulator 18. The firstpassivation layer 20 has a fourth contact hole 20 a that exposes aportion of the drain electrode 19 a. An anode electrode 21 that is madeof a transparent conductive material is disposed on the firstpassivation layer 20 and contacts the drain electrode 19 a through thefourth contact hole 20 a. A second passivation layer 22 is formed on theanode electrode 21 and on the exposed portions of the first passivationlayer 20. The second passivation layer 22 has a well 22 a that exposes aportion of the anode electrode 21. An electroluminescent layer 23 isformed on the second passivation layer 22 and into the well 22 a. Acathode electrode 24 is formed entirely over the surface including onthe exposed portions of the second passivation layer 22 and on theelectroluminescent layer 23. The cathode electrode 24 is formed of anopaque metallic conductive material.

In the active matrix organic electroluminescent display deviceillustrated in FIG. 2, the anode electrode 21 is formed of thetransparent conductive material, while the cathode electrode 24 isformed of the opaque conductive material. Thus, the light emitted fromthe organic electroluminescent layer 23 is released in a bottomdirection. Such device is called the bottom emission type.

FIGS. 3A to 3I are cross-sectional views illustrating a fabricatingprocess of the active matrix organic electroluminescent display deviceof FIG. 2. Many of the patterns shown in FIGS. 3A to 3G are formedthrough a photolithography process of photoresist (PR) coating,aligning, exposing and developing steps using a mask.

In FIG. 3A, after a buffer layer 11 is formed on an entire surface of asubstrate 10, first and second semiconductor layers 12 and 13 ofpolycrystalline silicon are formed on the buffer layer 11 through afirst mask process. The first and second polycrystalline semiconductorlayers 12 and 13 have island shapes.

In FIG. 3B, an insulator of silicon nitride or silicon oxide and aconductive material of metal are sequentially deposited on the firstpolycrystalline silicon layer 12 and then patterned using a second mask,thereby sequentially forming a gate insulation layer 12 and a gateelectrode 15 on the first polycrystalline semiconductor layer 12.Thereafter, impurities such as p-type ions are doped on the exposedportions of the first and second polycrystalline semiconductor layers 12and 13. During doping, the gate electrode 15 acts as a mask so that thefirst polycrystalline semiconductor layer 12 is divided into an activeregion 12 a where the impurities are not doped and drain and sourceregions 12 b and 12 c where the impurities are doped. Further, thesecond polycrystalline semiconductor layer 13 on which the impuritiesare fully doped becomes a capacitor electrode 13 a. The drain and sourceregions 12 b and 12 c are located on both sides of the active region 12a.

Referring to FIG. 3C, a first interlayer insulator 16 is formed on theentire surface of the buffer layer 11 so as to cover the gate electrode15, the drain and source regions 12 b and 12 c, and the capacitorelectrode 13 a. After forming the first interlayer insulator 16 over theentire surface of the substrate 10, a power line 17 of metal is formedthrough a third mask process on the first interlayer insulator 16particularly to overlap the capacitor electrode 13 a. Since the powerline 17 is formed right above the capacitor electrode 13 a, it forms astorage capacitor with the capacitor electrode 13 a and the interposedfirst interlayer insulator 16.

In FIG. 3D, a second interlayer insulator 18 is formed on the firstinterlayer insulator 16 and on the power line 17. Thereafter, first tothird contact holes 18 a, 18 b and 18 c are formed using a fourth maskprocess. The first contact hole 18 a exposes the drain region 12 b, thesecond contact hole 18 b exposes the source region 12 c, and the thirdcontact hole 18 c exposes the power line 17.

In FIG. 3E, a metal layer is formed on the second passivation layer 18and then patterned through a fifth mask process, thereby forming a drainelectrode 19 a and a source electrode 19 b. The drain electrode 19 acontacts the drain region 12 b through the first contact hole 18 a,while the source electrode 19 b contacts the source region 12 c throughthe second contact hole 18 b. Furthermore, the source electrode 19 bcontacts the power line 17 through the third contact hole 18 c.

Through the previously described process, a driving thin film transistorhaving the semiconductor layer 12, the gate electrode 15, the drain andsource electrodes 19 a and 19 b is completed. Moreover, a regioncorresponding to the power line 17 and the capacitor electrode 13 aforms the storage capacitor. Although not shown in FIG. 3E, but shown inFIG. 1, the storage electrode 13 is connected to the gate electrode 15of the driving thin film transistor, and the power line 17 is parallelto the signal line.

In FIG. 3F, a first passivation layer 20 having a fourth contact hole 20a resulting from a sixth mask process is formed on the second interlayerinsulator while covering the drain and source electrodes 19 a and 19 b.The fourth contact hole 20 a exposes a portion of the drain electrode 19a.

In FIG. 3G, a transparent conductive material is deposited on the firstpassivation layer 20 and then patterned using a seventh mask process,thereby forming an anode electrode 21 that contacts the drain electrode19 a through the fourth contact hole 20 a.

In FIG. 3H, a second passivation layer 22 is formed on the anodeelectrode 21 and on the exposed portion of the first passivation layer20. Thereafter, the second passivation layer 22 is patterned using aneighth mask process, thereby forming a well 22 a that exposes a portionof the anode electrode 21.

Now in FIG. 3I, an organic electroluminescent layer 23 is formed on thesecond passivation layer to contact the anode electrode 21 through thewell 22 a. Thereafter, a cathode electrode 24 is formed on the organicelectroluminescent layer 23 and on the exposed portion of the secondpassivation layer 22. The cathode electrode 24 entirely covers thesubstrate 10.

In the above-mentioned processes forming the organic electroluminescentdisplay device, a plurality of thin film depositions are repeated, andmoreover a plurality of photolithography processes that use masks arealso repeated many times. Therefore, these repetitions increase the maskprocess. Since the photolithography process includes a rinsing process,a photoresist deposition process, an exposure process, a developingprocess, an etching process, etc., the manufacturing time and the costof production can be reduced if only one mask process is omitted. Theorganic electroluminescent display device described with reference toFIGS. 3A to 3I, however, requires eight masks, resulting in a decreasedproduction yield and increased cost of production. Moreover, the moremasks the organic electroluminescent display device requires, the moredefects the fabrication process creates.

Additionally, since the active matrix organic electroluminescent displaydevice of the related art has a capacitor electrode that is an opaquematerial, it has a decreased luminant area and a reduced aperture ratio.In order to overcome these problems, the current density should beraised to increase the luminance of the device, thereby causing adecreased life span of the organic electroluminescent display device.

Furthermore, since the organic electro luminescent display device of therelated art has the power line in a shape of line, the power line iseasily deteriorated and damaged and the active matrix organicelectroluminescent display device do not display images uniformly.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an active matrixorganic electroluminescent display device that substantially obviatesone or more of problems due to limitations and disadvantages of therelated art.

An advantage of the present invention is to provide an active matrixorganic electroluminescent display device of low fabrication cost andhigh production yield by reducing the number of mask processes.

Another advantage of the present invention is to provide an activematrix organic electroluminescent display device that has a highaperture ratio and a long life span.

Another advantage of the present invention is to provide an activematrix organic electroluminescent display device that preventsdeterioration and damage of a power line and displays uniform images.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. These andother advantages of the invention will be realized and attained by thestructure particularly pointed out in the written description and claimshereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, an activematrix organic electroluminescent display device includes: a substrate;a ground layer on the substrate; a buffer layer on the ground layer; apolycrystalline semiconductor layer on the buffer layer, thepolycrystalline semiconductor layer having an active region, a drainregion and a source region, wherein the active region is disposed in themiddle of the polycrystalline silicon layer and the drain and sourceregions are disposed in both sides of the active region; a gateinsulation layer on the buffer layer to cover the polycrystallinesilicon layer; a gate electrode on the gate insulation layer, the gateelectrode disposed right above the active region of the polycrystallinesilicon layer; a first capacitor electrode on the gate insulation layer;an interlayer insulator formed on the gate insulation layer to cover thegate electrode and the first capacitor electrode; drain and sourceelectrodes on the interlayer insulator, the drain and source electrodecontacting the drain and source regions, respectively, through first andsecond contact holes that penetrate the interlayer insulator and thegate insulation layer; a cathode electrode formed on the interlayerinsulator and connected to the drain electrode; a second capacitorelectrode on the interlayer insulator; a passivation layer formed on theinterlayer insulator to cover the drain and source electrodes, thecathode electrode and the second capacitor electrode, the passivationlayer having a bank that exposes the cathode electrode; an organicelectroluminescent layer on the passivation layer and into the bank, theorganic electroluminescent layer contacting the cathode electrodethrough the bank; and an anode electrode on the exposed portion of thepassivation layer and on the organic electroluminescent layer.

In another aspect of the present invention, a fabricating method of anactive matrix organic electroluminescent display device includes:forming a ground layer on a substrate; forming a buffer layer on theground layer; forming a polycrystalline semiconductor layer on thebuffer layer; forming a gate insulation layer on the buffer layer tocover the polycrystalline silicon layer; forming a gate electrode and afirst capacitor electrode on the gate insulation layer, the gateelectrode disposed right above the polycrystalline silicon layer; dopingions into the polycrystalline semiconductor layer using the gateelectrode as a mask so that the polycrystalline semiconductor layer hasan active region, a drain region and a source region, wherein the activeregion is disposed in the middle of the polycrystalline silicon layerright under the gate electrode and the drain and source regions aredisposed in both sides of the active region; forming an interlayerinsulator on the gate insulation layer to cover the gate electrode andthe first capacitor electrode; forming first, second, third and fourthcontact holes, wherein the first and second contact holes expose thedrain and source regions, respectively, by penetrating both theinterlayer insulator and the gate insulation layer, and the third andfourth contact holes expose portions of the ground layer by penetratingthe interlayer insulator, the gate insulation layer and the bufferlayer; forming drain and source electrodes on the interlayer insulator,the drain and source electrode contacting the drain and source regions,respectively, through the first contact hole and through the secondcontact hole; forming a cathode electrode on the interlayer insulator,the cathode electrode connected to the drain electrode; forming a secondcapacitor electrode on the interlayer insulator; forming a passivationlayer on the interlayer insulator to cover the drain and sourceelectrodes, the cathode electrode and the second capacitor electrode,the passivation layer having a bank that exposes the cathode electrode;forming an organic electroluminescent layer on the passivation layer andinto the bank, the organic electroluminescent layer contacting thecathode electrode through the bank; and forming an anode electrode onthe exposed portion of the passivation layer and on the organicelectroluminescent layer.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 is an equivalent circuit diagram showing a basic pixel structureof a related art active matrix organic electroluminescent display (ELD)device;

FIG. 2 is a schematic cross-sectional view of a related art activematrix organic electroluminescent display device;

FIGS. 3A to 3I are cross-sectional views illustrating a fabricatingprocess of an active matrix organic electroluminescent display device ofFIG. 2.

FIG. 4 is a schematic cross-sectional view of an active matrix organicelectroluminescent display device according to an exemplary embodimentof the present invention;

FIG. 5 is an equivalent circuit diagram showing a basic pixel structureof an active matrix organic electroluminescent display (ELD) deviceaccording to the present invention;

FIG. 6 is a plan view schematically illustrating a power line and aground layer according to an exemplary embodiment of the presentinvention;

FIGS. 7A to 7F are cross-sectional views showing a fabricating processof an active matrix organic electroluminescent display device of FIG. 4;

FIG. 8 is a schematic cross-sectional view of an active matrix organicelectroluminescent display device according to another exemplaryembodiment of the present invention;

FIG. 9 is a photo showing a polycrystalline silicon layer that is formedabove the ground layer shown in FIG. 6;

FIG. 10 is a plan view schematically illustrating a ground layeraccording to another exemplary embodiment of the present invention; and

FIG. 11 is a photo showing a polycrystalline silicon layer that isformed above the ground layer shown in FIG. 10.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Reference will now be made in detail to an embodiment of the presentinvention, example of which is illustrated in the accompanying drawings.Wherever possible, similar reference numbers will be used throughout thedrawings to refer to the similar or like parts.

FIG. 4 is a schematic cross-sectional view of an active matrix organicelectroluminescent display (ELD) device according to an exemplaryembodiment of the present invention. Since the active matrix organic ELDdevice uses a p-Si thin film transistor (TFT), a top gate type isadopted.

In FIG. 4, a ground layer 120 is formed on entire surface of a substrate110. The ground layer 120 is a conductive material, such as metal. Abuffer layer 130 of silicon nitride or silicon oxide is formed on theground layer 120. A semiconductor layer 131 (132 and 133) that has anisland-shape is formed of polycrystalline silicon on the buffer layer130. The polycrystalline semiconductor layer is divided into an activeregion 131 to which dopant is not applied and drain and source regions132 and 133 to which the dopant is applied and doped. Here, the bufferlayer 130 prevents that the impurities infiltrating from the substrate110 or the ground layer 120 into the polycrystalline semiconductor layer131 (132 and 133). A gate insulation layer 140 is formed on the bufferlayer 130, covering the active, drain and source regions 131, 132 and133. A gate electrode 151 is formed on the gate insulation layer 140above the active region 131 of the polycrystalline semiconductor layer.The gate electrode 151 may be formed right above the active region 131.A first capacitor electrode 152 that is made of the same material as thegate electrode 151 is formed on the gate insulation layer 140.

Still referring to FIG. 4, an interlayer insulator 160 is disposed onthe gate insulation layer 140, covering the gate electrode 151 and thefirst capacitor electrode 152. Meanwhile, first and second contact holes161 and 162, which penetrate both the interlayer insulator 160 and thegate insulation layer 140, expose the drain region 132 and source region133, respectively. Additionally, third and fourth contact holes 163 and164, which penetrate the interlayer insulator 160, gate insulation layer140 and buffer layer 130, are formed and expose portions of the groundlayer 120. A cathode electrode 171, a drain electrode 172, a sourceelectrode 173 and a second capacitor electrode 174, which are made of anopaque conductive material, such as metal, are disposed on theinterlayer insulator 160. The cathode electrode 171 is connected to thedrain electrode 172 that contacts the drain region 132 through the firstcontact hole 161. The source electrode 173 contacts both the drainregion 133 and the ground layer 120, respectively, through the secondcontact hole 162 and through the third contact hole 163. The secondcapacitor electrode 174 contacts the ground line 120 through the fourthcontact hole 164. The first and second capacitor electrodes 152 and 174form a storage capacitor with the interposed interlayer insulator 160.Although not shown in FIG. 4, the first capacitor electrode 152 isconnected to the gate electrode 151 when viewed in a superficialobservation. A passivation layer 180 is formed on the interlayerinsulator 160 to cover the cathode electrode 171, the drain electrode172, the source electrode 173 and the second capacitor electrode 174.The passivation 180 has a well 181 that exposes the cathode electrode171. An electroluminescent layer 190 is formed on the passivation layer180 and into the well 181 so that the electroluminescent layer 190contacts the cathode electrode 171 through the well 181. An anodeelectrode 200 is formed on the electroluminescent layer 190 and on thepassivation layer 180. The anode electrode 200 is a transparentconductive material, such as indium tin oxide or indium zinc oxide. Theanode electrode 200 is disposed entirely over the substrate 110 so thatthe anode electrode 200 acts as a power line.

FIG. 5 is an equivalent circuit diagram showing a basic pixel structureof an active matrix organic electroluminescent display (ELD) deviceaccording to the present invention. As shown in FIG. 5, a gate line 212is arranged in a first direction, and a data line 211 is arranged in asecond direction substantially perpendicular to the first direction,thereby defining a pixel region. Near a crossing of the data and gatelines 211 and 212, a switching thin film transistor (TFT) 214 isdisposed and connected to the data and gate lines 211 and 212. The gateelectrode of the switching TFT 214 is connected to the gate line 212,and the source electrode of the switching TFT 214 is connected to thedata line 211. Furthermore, the switching TFT 214 is connected to both adriving thin film transistor (TFT) 215 and a storage capacitor 216.Namely, the drain electrode of the switching TFT 214 is connected toboth the gate electrode of the driving TFT 215 and the capacitorelectrode of the storage capacitor 216. The drain electrode of thedriving TFT 215 is connected to a cathode electrode of aelectroluminescent diode 217. The source electrode of the driving TFT215 is grounded. The anode electrode of the electroluminescent diode 217is connected to a power line 213. In order to uniformly maintain a gatevoltage of the driving TFT 215, the storage capacitor 216 is connectedto both the gate and source electrodes of the driving TFT 215.

In the active matrix organic electroluminescent display deviceillustrated in FIG. 5, it is advisable that the driving TFT 216 is ann-type thin film transistor. However, the switching TFT 215 can beeither an n-type thin film transistor or a p-type thin film transistor.

As described with reference to FIG. 4, the ground layer and thepower-line layer are disposed all over the substrate. FIG. 6 is a planview schematically illustrating the ground layer 212 and the power-linelayer 222 according to an exemplary embodiment of the present invention.In FIG. 6, the thin film transistors and the storage capacitors areomitted for simple illustration.

As illustrated in FIG. 6, the ground line 221 and the power-line layer222 are formed over the substrate 220. The ground layer 221 is disposedall over the substrate 220, but some portions of the substrate areexposed. Additionally, the power-line layer 222 is also disposed allover the substrate 220 so that some portions of the substrate 220 areexposed. The overlapped area of the ground line 221 and power-line layer222 is a display region where the images appear, and thus a plurality ofthin film transistors and a plurality of electroluminescent diodes aredisposed in that display region. In the present invention, thepower-line layer 222 acts as the anode electrode of theelectroluminescent diode, as described before.

According to the present invention, since the ground layer and thepower-line layer are formed all over the substrate, the resistance ofthe power line is reduced and the thermal damage that may be caused inthe power line during driving the device is prevented. Therefore, theimage quality increases and the uniformity in the display can beobtained.

In the first embodiment illustrated in FIG. 4, the cathode electrode isformed of the opaque conductive material, and the anode electrode isformed of the transparent conductive material. Thus, the organicelectroluminescent display device can be a top emission type in whichthe light is released in an upper direction. A high aperture ratio canbe obtained. As a result, the luminance of the display device canincrease although the current density is not large. It isdistinguishable in the present invention that the life span of theorganic electroluminescent display device becomes lengthened.

FIGS. 7A to 7F are cross-sectional views illustrating a fabricatingprocess of the active matrix organic electroluminescent display deviceof FIG. 4.

In FIG. 7A, a conductive material, such as metal, is deposited on asubstrate 110 and then patterned using a first mask process to form aground line 120. When viewed in a superficial observation, the groundline 120 has the shape shown in FIG. 6 so that it is widely disposedover the substrate 110 and covers the display region where the imagesappear. Thereafter, a buffer layer 130 is formed on the ground line 120.The buffer layer 130 is one of silicon oxide and silicon nitride. Next,a polycrystalline silicon layer is formed on an entire surface of thebuffer layer 130 and then patterned using a second mask process to forma semiconductor layer 135. There are many ways to form thepolycrystalline silicon. One is forming an amorphous silicon layer onthe buffer layer and then heat-treating the amorphous silicon to formthe polycrystalline silicon. Another is that a laser irradiation is usedto convert the amorphous silicon into the polycrystalline silicon. Inthe present invention, the substrate 110 can be a glass or othertransparent substances.

In FIG. 7B, a gate insulation layer 140 of silicon nitride or siliconoxide is formed on the buffer layer 130 to cover the semiconductor layer135, and then a metallic material is deposited on the gate insulationlayer 140. The deposited metallic material is patterned using a thirdmask process to form a gate electrode 151 and a first capacitorelectrode 152. The gate electrode 151 is disposed above thesemiconductor layer 135. Using the gate electrode 151 as a mask, thedopant (e.g., n-type ions) is applied to and doped on portions of thesemiconductor layer 135. Thus, the semiconductor layer 135 is dividedinto an active region 131 in the middle and drain and source regions 132and 133 on both sides of the active region 131. Because the gateelectrode 151 acts as a mask during the doping process, the dopant doesnot exist in the active region 131, but only exists in the drain andsource regions 132 and 133. Although not shown in FIG. 7B, the firstcapacitor electrode 152 is electrically connected to the gate electrode151.

In FIG. 7C, an interlayer insulator 160 is formed on the gate insulationlayer 140 so as to cover the gate electrode 151 and the first capacitorelectrode 152. Thereafter, a fourth mask process is performed to formfirst to fourth contact holes 161, 162, 163 and 164. The first andsecond contact holes 161 and 162 penetrate both the interlayer insulatorand the gate insulation layer 140, and respectively expose the drainregion 132 and the source region 133. Additionally, the third and fourthcontact holes 163 and 164 penetrate the interlayer insulator 160, thegate insulation layer 140 and the buffer layer 130 so that they exposeportions of the ground layer 120.

In FIG. 7D, a conductive material, such as metal, is deposited on theinterlayer insulator 160 and then patterned using a fifth mask processso as to form a cathode electrode 171, a drain electrode 172, a sourceelectrode 173 and a second capacitor electrode 174. The cathodeelectrode 171 is connected to the drain electrode 172, as shown in FIG.7D. The drain electrode 172 contacts the drain region 132 through thefirst contact hole 161. The source electrode 173 contacts the sourceregion 133 and the ground line 120 through the second contact hole 162and through the third contact hole 163, respectively. Furthermore, thesecond capacitor electrode 174 contacts the ground line 120 through thefourth contact hole 164 and forms the storage capacitor with the firstcapacitor electrode 152 and the interposed interlayer insulator 160.

Now in FIG. 7E, a passivation layer 180 is formed on the patternedconductive layer and on the exposed portions of the interlayer insulator160. Thus, the passivation layer 180 covers the cathode electrode 171,the drain electrode 172, the source electrode 173 and the secondcapacitor electrode 174. Thereafter, a portion of the passivation layer180 is patterned through a sixth mask process, thereby resulting in awell 181 that exposes the cathode electrode 171.

In FIG. 7F, an organic electroluminescent layer 190 is formed on thepassivation layer 180 and on the exposed cathode electrode 171. Theorganic electroluminescent layer 190 contacts the cathode electrode 171through the well 181. Thereafter, a transparent conductive material,such as indium tin oxide or indium zinc oxide, is formed on the organicelectroluminescent layer 190 and on the passivation layer 180, therebyforming an anode electrode 200. At the time of forming the organicelectroluminescent layer 190, an inkjet method or a shadow mask is usedso that an additional mask process is not required. Furthermore, sincethe shadow mask is also used to form the anode electrode 200, theadditional mask process is not required either.

As described with reference to FIGS. 7A to 7F, the active matrix organicelectroluminescent display device of the present invention is fabricatedthrough the first to sixth mask processes. Thus, the fabrication timeand the cost of production are dramatically reduced as compared with therelated art. Furthermore, it is possible that the defects of the layerelements is lessened because the reduction of the mask process.Accordingly, the fabrication yield increases in the present invention.

The active matrix organic electroluminescent display device shown inFIGS. 4 and 7A-7F is the top emission type. However, the principles ofthe present invention can be adopted in the bottom emission type device.The active matrix organic electroluminescent display device of thebottom emission type will be described in detail with reference to FIG.8.

FIG. 8 is a schematic cross-sectional view of an active matrix organicelectroluminescent display device according to another exemplaryembodiment of the present invention.

As shown in FIG. 8, a ground line 320 of a transparent conductivematerial is entirely formed on a substrate 310. The transparentconductive material may be indium tin oxide (ITO) or indium zinc oxide(IZO) or the like, and the substrate 310 is substantially a glass, forexample. A buffer layer 330 is formed on the ground line 320. Asemiconductor layer having an active region 331, drain region 332 andsource region 333 is formed on the buffer layer 330. The semiconductorlayer is formed of polycrystalline silicon and shaped like an island.The active layer 331 is a pure silicon region to which no dopant isapplied. The drain and source regions 332 and 333 disposed on both sidesof the active layer 331 are impurity-doped regions where the dopant isapplied and doped.

Thereafter, a gate insulation layer 340 is formed on the buffer layer330, covering the active, drain and source regions 331, 332 and 333. Agate electrode 351 is formed on the gate insulation layer 340particularly right the active region 331 of the polycrystallinesemiconductor layer. The gate electrode 351 may be formed right abovethe active region 331. Further, a first capacitor electrode 352 that ismade of the same material as the gate electrode 351 is formed on thegate insulation layer 340. Although not shown in FIG. 8, the gateelectrode 351 and the first capacitor electrode 352 are electricallyconnected to each other. Next, an interlayer insulator 360 is disposedon the gate insulation layer 340, covering the gate electrode 351 andthe first capacitor electrode 352.

Meanwhile, first and second contact holes 361 and 362, which penetrateboth the interlayer insulator 360 and the gate insulation layer 340, areformed to expose the drain region 132 and source region 133,respectively. Additionally with the first and second contact holes 361and 362, third and fourth contact holes 163 and 164, which penetrate theinterlayer insulator 160, gate insulation layer 140 and buffer layer130, are formed to expose portions of the ground layer 320. A cathodeelectrode 371, a drain electrode 372, a source electrode 373 and asecond capacitor electrode 374 are disposed on the interlayer insulator360. The cathode electrode 371 has a single-layered structure, while thedrain electrode 372, the source electrode 373 and the second capacitorelectrode 374 have a double-layered structure. The cathode electrode 371is made of a transparent conductive material, such as indium tin oxide(ITO) or indium zinc oxide (IZO), for example. Further, the lower part372 a of the drain electrode 372, the lower part 373 a of the sourceelectrode 373 and the lower part 374 a of the second capacitor electrode374 are also made of the transparent conductive material, for example,indium tin oxide (ITO) or indium zinc oxide (IZO). On the contrary, theupper part 372 b of the drain electrode 372, the upper part 373 b of thesource electrode 373 and the upper part 374 b of the second capacitorelectrode 374 are made of an opaque conductive material, such as metal,for example. The cathode electrode 371 is connected to the lower part372 a of the double-layered drain electrode 372, which contacts thedrain region 332 through the first contact hole 361. The lower part 373a of the source electrode 373 contacts both the drain region 333 and theground layer 320 through the second contact hole 362 and through thethird contact hole 363, respectively. The lower part 374 a of the secondcapacitor electrode 374 contacts the ground line 320 through the fourthcontact hole 364. The first and second capacitor electrodes 352 and 374form a storage capacitor with the interposed interlayer insulator 360.

Next, a passivation layer 380 is formed on the interlayer insulator 360to cover the cathode electrode 371, the drain electrode 372, the sourceelectrode 373 and the second capacitor electrode 374. The passivationlayer 380 has a well 381 that exposes the cathode electrode 371. Anelectroluminescent layer 390 is then formed on the passivation layer 380and into the well 381 so that the electroluminescent layer 390 contactsthe cathode electrode 371 through the well 381. An anode electrode 400is formed on the electroluminescent layer 390 and on the passivationlayer 380. Here, it is distinguishable that the anode electrode 400 isformed of an opaque conductive material, such as metal, for example. Theanode electrode 400 is disposed entirely over the substrate 310 so thatthe anode electrode 400 acts as a power line.

In the exemplary example of the organic electroluminescent displaydevice shown in FIG. 8, the ground layer 320 and the power-line anodelayer 400 are formed all over the substrate 310. Thus, the resistance ofthe power line is reduced, and the thermal damage that may occur in thepower line during driving the device is prevented. Therefore, the imagequality increases and the uniformity in the display can be obtained.Moreover, since the ground line 320 and the cathode electrode 371 aremade of the transparent conductive layer and the anode electrode 400acting as the power line is made of the opaque conductive material, theactive matrix organic electro luminescent display device of FIG. 8becomes the bottom emission type in which the light is released in thebottom direction.

When fabricating the active matrix organic electroluminescent displaydevice, a six-mask process can be adopted as like the first embodimentof FIGS. 7A to 7F. In order to use only six masks in the fabrication ofthe device shown in FIG. 8, the double layers of the transparentconductive material and the opaque conductive material are sequentiallyformed on the interlayer insulator 360. Thereafter, an exposure processis conducted using a special mask that has a slit in a portion for thecathode electrode 371. Thus, the six-mask process is available eventhough the drain and source electrodes 372 and 373 and the secondcapacitor electrode 374 have the double-layered structure.

Meanwhile, the polycrystalline semiconductor layer having the active,drain and source regions are formed by applying heat or laser beam tothe deposited amorphous silicon layer. However, since the ground layeris disposed on the entire of the substrate and then the amorphoussilicon layer is formed above the ground layer in the above-mentionedstructures, the amorphous silicon layer is not properly crystallizedbecause the applied heat for crystallization is dispersed by groundlayer having high thermal conductivity. This heat dispersion phenomenoninduces a long crystallization time and produces inappropriatepolycrystalline silicon. The polycrystalline silicon layer formed rightabove the ground layer is shown in FIG. 9. As shown in FIG. 9, thegrains of the polycrystalline silicon are relatively small so that thethin film transistor having these small grain sizes does not have goodelectrical properties and characteristics. Specifically, when using thelaser beam for crystallization, the crystallization becomes worsebecause the light energy of the laser beam more easily disperses throughthe ground layer.

Therefore, to overcome this problem of thermal or light energydispersion, another embodiment of the present invention provides aground layer that has a plurality of openings each corresponding inposition to the semiconductor layer. The ground layer having theplurality of openings is illustrated in FIG. 10. The ground layer 421has the plurality of openings 421 a therein in a row-and-columnformation. Each opening 421 a corresponds to a thin film transistor,especially to the semiconductor layer having the active, drain andsource regions. When using the ground layer 421 of FIG. 10, thesemiconductor layer of polycrystalline silicon can have the largegrains.

FIG. 11 is a photo showing the polycrystalline silicon layer that isformed above the ground layer shown in FIG. 10. Due to the opening 421 aof the ground layer 421, the amorphous silicon is not deprived of thethermal or light energy when the amorphous silicon is crystallized.Thus, the grains of polycrystalline semiconductor layer become larger asshown in FIG. 11. The thin film transistor having this large-sizedpolycrystalline silicon semiconductor layer can have the good electricalproperties and characteristics.

The illustrated embodiments of the present invention have the followingadvantages.

First, since the ground line and the power line are entirely disposedabove the substrate, the resistance of the power line is reduced and thethermal damage that may occur in the power line during driving thedevice is prevented. Therefore, the image quality increases and theuniformity in the display can be obtained.

Second, since the cathode electrode are formed with the drain and sourceelectrode at the same time, the fabrication process can be reduced andthe cost of production is lowered. Further, defect occurrence is reduceddue to the process reduction, and the production yield can be raised.

Third, the principles of the present invention can be applied to eitherthe top emission type organic electroluminescent display device or thebottom emission type organic electroluminescent display device. When itis utilized for the top emission type, the active matrix organicelectroluminescent display device can have a high aperture ratio.

It will be apparent to those skilled in the art that variousmodifications and variation can be made in the present invention withoutdeparting from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. An active matrix organic electroluminescent device, comprising: asubstrate; a ground layer on the substrate; a buffer layer on the groundlayer; a polycrystalline semiconductor layer on the buffer layer, thepolycrystalline semiconductor layer having an active region, a drainregion and a source region, wherein the active region is in the middleof the polycrystalline silicon layer and the drain and source regionsare disposed in both sides of the active region; a gate insulation layeron the buffer layer to cover the polycrystalline silicon layer; a gateelectrode on the gate insulation layer, above the active region of thepolycrystalline silicon layer; a first capacitor electrode on the gateinsulation layer; an interlayer insulator on the gate insulation layerto cover the gate electrode and the first capacitor electrode; drain andsource electrodes on the interlayer insulator, the drain and sourceelectrode contacting the drain and source regions, respectively, throughfirst and second contact holes that penetrate the interlayer insulatorand the gate insulation layer; a cathode electrode on the interlayerinsulator, the cathode electrode extended from and integrally formed asone unit with the drain electrode; a second capacitor electrode on theinterlayer insulator; a passivation layer on the interlayer insulator tocover the drain and source electrodes, the cathode electrode and thesecond capacitor electrode, the passivation layer having a well thatexposes the cathode electrode; an organic electroluminescent layer onthe passivation layer and into the well, the organic electroluminescentlayer contacting the cathode electrode through the well; and an anodeelectrode on the exposed portion of the passivation layer and on theorganic electroluminescent layer.
 2. The device according to claim 1,wherein the gate electrode is electrically connected to the firstcapacitor electrode.
 3. The device according to claim 2, wherein thegate electrode and the first capacitor electrode are of the samematerial.
 4. The device according to claim 1, wherein the sourceelectrode contacts the ground line through a third contact hole thatpenetrates the interlayer insulator, the gate insulation layer and thebuffer layer to expose a portion of the ground layer.
 5. The deviceaccording to claim 1, wherein the second capacitor electrode contactsthe ground line through a fourth contact hole that penetrates theinterlayer insulator, the gate insulation layer and the buffer layer toexpose a portion of the ground layer.
 6. The device according to claim1, wherein the anode electrode is all over the substrate and acts as apower line.
 7. The device according to claim 1, wherein the first andsecond capacitor electrodes with the interlayer insulator layerinterposed therebetween form a storage capacitor.
 8. The deviceaccording to claim 1, wherein the drain and source regions of thepolycrystalline semiconductor layer are doped with ions, and the activeregion is pure silicon.
 9. The device according to claim 1, wherein theground layer has a plurality of openings each corresponding in positionto the polycrystalline semiconductor layer.
 10. The device according toclaim 1, wherein the ground layer is an opaque conductive material. 11.The device according to claim 10, wherein the opaque conductive materialis metal.
 12. The device according to claim 10, wherein the cathodeelectrode, the drain electrode, the source electrode and the secondcapacitor electrode are the same material of an opaque conductivematerial.
 13. The device according to claim 12, wherein the opaqueconductive material is metal.
 14. The device according to claim 10,wherein the anode electrode is formed of a transparent conductivematerial.
 15. The device according to claim 14, wherein the transparentconductive material includes one of indium tin oxide and indium zincoxide.
 16. The device according to claim 1, wherein the ground layer isformed of a transparent conductive material.
 17. The device according toclaim 16, wherein the transparent conductive material is one of indiumtin oxide and indium zinc oxide.
 18. The device according to claim 16,wherein the cathode electrode is formed of a transparent conductivematerial.
 19. The device according to claim 18, wherein the transparentconductive material is one of indium tin oxide and indium zinc oxide.20. The device according to claim 16, wherein the anode electrode is anopaque conductive material.
 21. The device according to claim 20,wherein the opaque conductive material is metal.
 22. The deviceaccording to claim 16, wherein the drain electrode, the source electrodeand the second capacitor electrode have a double-layered structure of atransparent conductive material and an opaque conductive material. 23.The device according to claim 22, wherein the transparent conductivematerial includes one of indium tin oxide and indium zinc oxide, and theopaque conductive material includes metal.
 24. The device according toclaim 1, wherein the gate electrode is right above the active region ofthe polycrystalline silicon layer.